National Aeronautics and and Space Space Administration Administration 0 300,000,000 900,000,000 1,500,000,000 2,100,000,000 2,700,000,000 3,300,000,000 3,900,000,000 4,500,000,000 5,100,000,000 5,700,000,000 kilometers Our Star — The Sun www.nasa.gov Our solar system’s central star, the Sun, has inspired mythologi- region. Sunspot numbers fluctuate every 11 years as part of the SIGNIFICANT DATES Sun’s magnetic activity cycle. Also connected to this cycle are cal stories in cultures around the world, including those of the 150 CE — Greek scholar Claudius Ptolemy writes the bright solar flares and huge coronal mass ejections that blast off ancient Egyptians, the Aztecs of México, Native American tribes Almagest, formalizing the Earth-centered model of the solar of the Sun. of North America and Canada, the Chinese, and many others. system. The model was accepted until the 16th century. A number of ancient cultures built stone structures or modified The temperature of the photosphere is about 5,500 degrees 1543 — Nicolaus Copernicus publishes On the Revolutions of natural rock formations to mark the motions of the Sun and Celsius (10,000 degrees Fahrenheit). Above the photosphere lie the Celestial Spheres describing his heliocentric (Sun-centered) Moon — they charted the seasons, created calendars, and the tenuous chromosphere and the corona (“crown”). Visible light model of the solar system. monitored solar and lunar eclipses. These architectural sites from these top regions is usually too weak to be seen against the 1610 — First observations of sunspots through a telescope show evidence of deliberate alignments to astronomical phe- brighter photosphere, but during total solar eclipses, when the made independently by Galileo Galilei and Thomas Harriot. nomena: sunrises, moonrises, moonsets, even stars or planets. Moon covers the photosphere, the chromosphere can be seen 1645–1715 — Sunspot activity declines to almost zero, possibly Many cultures believed that the Earth was immovable and the as a red rim around the Sun while the corona forms a beauti- causing a “Little Ice Age” on Earth. Sun, other planets, and stars revolved about it. Ancient Greek ful white crown with plasma streaming outward, forming the 1860 — Eclipse observers see a massive burst of material from astronomers and philosophers knew this “geocentric” concept “points” of the crown. the Sun; it is the first recorded coronal mass ejection. from as early as the 6th century BCE. Now we know, of course, 1994 — The Ulysses spacecraft makes the first observations of that all the planets orbit our lone star — the Sun. Above the photosphere, temperature increases with altitude, the Sun’s polar regions. reaching as high as 2 million degrees Celsius (3.5 million degrees The Sun is the closest star to Earth, at a mean distance from 2004 — NASA’s Genesis spacecraft returns samples of the solar Fahrenheit). The source of coronal heating has been a scientific our planet of 149.60 million kilometers (92.96 million miles). This wind to Earth for study. mystery for more than 50 years. Likely solutions emerged from distance is known as an astronomical unit (abbreviated AU), and 2007 — NASA’s double-spacecraft STEREO mission returns the observations by the Solar and Heliospheric Observatory (SOHO) sets the scale for measuring distances all across the solar sys- first three-dimensional images of the Sun. and the Transition Region and Coronal Explorer (TRACE) mis- tem. The Sun, a huge sphere of mostly ionized gas, supports life 2009 — After more than 18 years, the Ulysses mission ends. sions, but the complete answer still evades scientists. Recent on Earth. The connection and interactions between the Sun and 2010 — SDO is launched and begins observing the Sun in missions — Hinode, Solar Terrestrial Relations Observatory Earth drive the seasons, ocean currents, weather, and climate. super-high definition. (STEREO), and the Solar Dynamics Observatory (SDO) — greatly 2011 — The STEREO spacecraft, from their dual perspective, About one million Earths could fit inside the Sun. It is held to- improved our knowledge of the corona, getting us still closer to see the entire Sun for the first time. gether by gravitational attraction, producing immense pressure the answer. They also give us an unprecedented understanding and temperature at its core. The Sun has six regions — the core, of the physics of space weather phenomena such as solar flares, ABOUT THE IMAGES coronal mass ejections, and solar energetic particles. Space the radiative zone, and the convective zone in the interior; the 1 2 1 Active regions spin visible surface (the photosphere); the chromosphere; and the weather can adversely affect our technology in space and on out bright loops above outermost region, the corona. The Sun has no solid surface. Earth; these missions help us to develop space weather reports. the Sun that trace 3 magnetic field lines At the core, the temperature is about 15 million degrees Celsius FAST FACTS (SDO image in extreme (about 27 million degrees Fahrenheit), which is sufficient to 4 5 sustain thermonuclear fusion. The energy produced in the core Spectral Type of Star G2V ultraviolet light). Age 4.6 billion years powers the Sun and produces essentially all the heat and light 2 Magnetic fields are believed to cause huge, super-hot Mean Distance to Earth 149.60 million km we receive on Earth. Energy from the core is carried outward by coronal loops that tower above the Sun’s surface (TRACE image). radiation, which bounces around the radiative zone, taking about (92.96 million mi) (1 astronomical unit) 3 An illustration of a coronal mass ejection and interaction 170,000 years to get from the core to the convective zone. The Rotation Period at Equator 26.8 days with Earth’s magnetic field (not to scale). The pressure from the temperature drops below 2 million degrees Celsius (3.5 million Rotation Period at Poles 36 days Sun forces Earth’s magnetic field into a windsock shape. degrees Fahrenheit) in the convective zone, where large bubbles Equatorial Radius 695,500 km (432,200 mi) 30 of hot plasma (a soup of ionized atoms) move upwards. Mass 1.989 × 10 kg 4 The Sun unleashed a solar flare with a spectacular coronal Density 1.409 g/cm3 mass ejection on June 7, 2011 (SDO extreme ultraviolet image). The Sun’s “surface” — the photosphere — is a 500-kilometer- Composition 92.1% hydrogen, 7.8% helium, 5 These large sunspots in the photosphere were associated thick (300-mile-thick) region, from which most of the Sun’s 0.1% other elements with several powerful solar flares in 2003 (SOHO image). radiation escapes outward and is detected as the sunlight we Surface Temperature (Photosphere) 5,500 deg C observe here on Earth about eight minutes after it leaves the (10,000 deg F) FOR MORE INFORMATION Sun. Sunspots in the photosphere are areas with strong mag- Luminosity* 3.83 × 1033 ergs/sec netic fields that are cooler, and thus darker, than the surrounding solarsystem.nasa.gov/sun *The total energy radiated by the Sun (or any star) per second at all wavelengths. LG-2013-07-564-HQ — JPL 400-1489C 07/13.